Try making one of your books free on Smashwords so Barnes & Noble will also have it free. That way, you may attract some new readers to giving it a chance and hopefully, they'll buy more of your other books.

Another vote for Smashwords. While they may leave something to be desired in formatting for each eReader, they can't be beaten in terms of distribution. You can get your work into Apple's ibookstore, Kobo, B&N, Scrollmotion and more to follow.